etcdserver: fix panic when checking IsLearner of removed member

[jscissr]

Previously, calling s.IsLearner() when the local node is no longer a member panics. There was an attempt to fix this by first checking IsMemberExist(), but this is not a correct fix because the member could be removed between the two calls. Instead of panicking when the member was removed, IsLearner() should return false. A node which is not a member is also not a learner.

There was a similar concurrency bug when accessing the IsLearner property of a member, which will panic with a nil pointer access error if the member is removed between the IsMemberExist() and Member() calls.

I did not add a unit test because it's basically impossible to test for such concurrency bugs.

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[k8s-ci-robot]

Hi @jscissr. Thanks for your PR.

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[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 68.79%. Comparing base (ce07474) to head (448ef94).
Report is 2 commits behind head on main.

❗ Current head 448ef94 differs from pull request most recent head d61be85

Please upload reports for the commit d61be85 to get more accurate results.

Files with missing lines	Patch %	Lines
server/etcdserver/api/membership/cluster.go	0.00%	1 Missing ⚠️
server/etcdserver/server.go	75.00%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

Files with missing lines	Coverage Δ
server/etcdserver/api/v3rpc/interceptor.go	77.60% <100.00%> (+3.12%)	⬆️
server/etcdserver/api/membership/cluster.go	88.27% <0.00%> (-0.26%)	⬇️
server/etcdserver/server.go	81.28% <75.00%> (-0.36%)	⬇️

... and 17 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #18606      +/-   ##
==========================================
- Coverage   68.81%   68.79%   -0.02%     
==========================================
  Files         420      420              
  Lines       35519    35516       -3     
==========================================
- Hits        24441    24434       -7     
  Misses       9648     9648              
- Partials     1430     1434       +4     

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